1. Field of the Invention
The present invention relates to an autofocus adjustment technique of an imaging apparatus having an imaging optical system that varies an amount of field curvature depending on an imaging distance.
2. Description of the Related Art
In recent years, suppression of residual aberrations in optical design of image-taking lenses has become more difficult in imaging apparatuses such as compact digital cameras as a result of the requirements for miniaturization, thinning, high magnification of zoom lenses, and the like. In general, a variation in aberrations occurs in imaging optical systems if image-taking conditions such as the distance to an object or the like change, resulting in a change in performance. Thus, a balanced design is provided such that the variation in aberrations falls within a predetermined range. For example, zoom lenses are designed in consideration of sufficient correction of aberrations at all imaging distances which are assumed to be used.
On the other hand, there is a tradeoff relationship between the degree of freedom in design and specification such as size, brightness, zoom magnification, and the like of imaging optical systems or degree of aberration correction. An increase in the degree of freedom in design is not readily achieved due to limitations such as design specification, the number or thickness of lenses, opening diameter, selection of glass materials, and the like. For example, among the aberrations to be corrected, field curvature varies depending on an imaging distance. If miniaturization of apparatuses advances, the remaining amount of aberration may not be negligible. An image may be affected in a macro imaging area due to the readily occurrence of field curvature toward the under direction upon image-taking at close-range distance. A field curvature may locally be increased at a specific middle zoom position as a matter of design. Furthermore, field curvature toward the over direction may occur on the telescopic side.
In the conventional imaging apparatuses, an image needs to be taken in an optimum focused state for various types of objects by mitigating the effect of axially asymmetric deformation of an image plane due to a variation in distance of field curvature components in terms of lens design and manufacturing error of lenses. In other words, image plane-related error components need to be removed upon acquiring distance information to a plurality of objects. Array determination or facing determination of the respective objects using distance information including error components may cause a trouble for image capture in an optimum focused state.